Method for producing formaldehyde from methanol

ABSTRACT

In a process for preparing formaldehyde from methanol by dehydrogenation in a reactor in the presence of a catalyst at a temperature in the range from 300 to 1000° C., a circulating gas stream comprising by-products of the dehydrogenation is passed through the reactor.

A number of processes for preparing formaldehyde from methanol are known(see, for example, Ullmann's Encyclopaedia of Industrial Chemistry). Theprocesses carried out industrially are predominantly the oxidation:

CH₃OH+½O₂→CH₂O+H₂O

over catalysts comprising iron oxide and molybdenum oxide at from 300°C. to 450° C. (Formox process) and the oxidative dehydrogenation (silvercatalyst process) according to:

CH₃OH→CH₂O+H₂

H₂+½O₂→H₂O

at from 600° C. to 720° C. In both processes, the formaldehyde is firstobtained as an aqueous solution. Particularly when used for thepreparation of formaldehyde polymers and oligomers, the resultingformaldehyde has to be subjected to costly dewatering. A furtherdisadvantage is the formation, as by-product, of the corrosive formicacid which has an adverse effect on the polymerization.

The dehydrogenation of methanol enables these disadvantages to beavoided and, in contrast to the abovementioned processes, virtuallywater-free formaldehyde to be obtained directly:

In order to achieve an ecologically and economically interestingindustrial process for the dehydrogenation of methanol, the followingprerequisites have to be met: The strongly endothermic reaction has tobe carried out at high temperatures so as to be able to achieve highconversions. Competing secondary reactions have to be suppressed inorder to achieve satisfactory selectivity to formaldehyde (withoutcatalysis, the selectivity for forming formaldehyde is less than 10% atconversions over 90%). The residence times have to be short and thecooling of the reaction products has to be rapid in order to lessen thedecomposition of the formaldehyde which is not thermodynamically stableunder the reaction conditions:

CH₂O→CO+H₂

Various methods of carrying out this reaction have been proposed; thus,for example, DE-A-37 19 055 describes a process for preparingformaldehyde from methanol by dehydrogenation in the presence of acatalyst at elevated temperature. The reaction is carried out in thepresence of a catalyst comprising at least one sodium compound at atemperature of from 300° C. to 800° C.

J. Sauer and G. Emig (Chem. Eng. Technol. 1995, 18, 284-291) were ableto set free a catalytically active species, presumed by them to besodium, from a catalyst comprising NaAlO₂ and LiAlO₂ by means of areducing gas mixture (87%N₂+13%H₂). This species can catalyze thedehydrogenation of methanol added downstream in the same reactor, i.e.methanol which does not come into contact with the catalyst bed, to giveformaldehyde. When using nonreducing gases, only a low catalyticactivity was observed.

According to J. Sauer and G. Emig and also results from more recentstudies (see, for example, M. Bender et al., Presentation to the XXX.Jahrestreffen deutscher Katalytiker, Mar. 21-23, 1997), sodium atoms andNaO molecules have been identified as species emitted into the gas phaseand their catalytic activity for the dehydrogenation of methanol in thegas phase has been described.

In the known processes, the starting material methanol is always reacteddiluted with nitrogen and/or nitrogen/hydrogen mixtures.

Although good results are already achieved using the known processes,there is still a wide scope for improvements from a technical andeconomic point of view.

In various documents, for example EP-A 0 130 068, EP-A 0 261 867 andDE-A 25 25 174, it is proposed that the gas mixture formed in thereaction be used as fuel after separating off the formaldehyde.

It has now surprisingly been found that a reaction procedure which isgreatly improved from a technical and economic point of view,particularly in terms of energy, can be achieved if the gas mixtureformed in addition to the formaldehyde is used for diluting the startingmaterial methanol.

The invention accordingly provides a process for preparing formaldehydefrom methanol by dehydrogenation in a reactor in the presence of acatalyst at a temperature in the range from 300 to 1000° C., wherein acirculating gas stream comprising by-products of the dehydrogenation ispassed through the reactor.

The process of the invention is an ecologically and economicallyfavorable method of producing formaldehyde having a low water content.The utilization of the hydrogen-rich by-products of the reaction, i.e.the product gas after separating off the formaldehyde, for diluting thestarting material methanol for the dehydrogenation enables, on the onehand, particularly high yields to be achieved and, on the other hand,owing to the good thermal conductivity, allows the outlay in terms ofapparatus for heating the starting materials, introducing the heat ofreaction and cooling the products to be minimized. The further possibleutilization of parts of the by-products of the reaction, i.e. theproduct gas after separating off the formaldehyde, as fuel forgenerating the reaction temperature necessary for the dehydrogenation,and also heat recovery from the waste gases, can provide the heat forthis and further process steps. In this case, essentially only thedesired product formaldehyde and the combustion products CO₂ and H₂Oleave the process.

For the purposes of the invention, dehydrogenation is a non-oxidativeprocess according to the equation:

For the purposes of the invention, the term “by-products” refers to thegas mixture which remains after separating off the product formaldehydeand comprises, apart from hydrogen, usually CO, CH₄ and CO₂ as well aspossibly CH₂O, MeOH, H₂O, HCOOCH₃ and/or residues from the separatingoff of formaldehyde, and preferably consists essentially of these gases.The ratio H₂/CO in the circulating gas is particularly preferably ≧3.

FIG. 1 shows a schematic overview of a preferred variant of the processof the invention.

Methanol 2 is taken from a reservoir 1, diluted with circulating gag 8,preheated in a heat exchanger 3 and introduced into the reactor 4. Aftergoing through the reactor 4, the gas is cooled in a heat exchanger 3′and the product mixture 5 is separated in the separation vessel 6 intoformaldehyde 7 and by-products 8 (circulating gas). At least part of theby-products is recirculated to the reactor by means of a conveyingdevice 9, for example a fan. A part of the by-products can afterdischarge be used directly as fuel in an apparatus 10 for firing thereaction vessel 4. The heat exchangers 3, 3′ can be a single unit.

The invention further provides an apparatus for carrying out theabovementioned process comprising a heat exchanger for preheating thestarting materials, a reactor for carrying out the dehydrogenation, aheat exchanger for cooling the product mixture, a separation vessel forseparating off the formaldehyde and also means, in particular a fan, forrecirculating at least part of the by-products of the reaction to thereactor.

In a preferred embodiment of the apparatus of the invention, theapparatus further comprises means for discharging a further part of theby-products of the dehydrogenation and for feeding this part to anapparatus for heating the reactor in which latter apparatus it serves asfuel.

Commercial methanol can be used for the reaction; it should preferablybe low in water and contain no substances which poison the catalyst.

To carry out the dehydrogenation, the fluid, preferably gaseous,methanol is diluted with gaseous by-products of the dehydrogenation.

The molar proportion of methanol is generally from 5 to 90%, preferablyfrom 10 to 50%, particularly preferably from 10 to 40%. The amount ofcirculating gas required follows from the proportion of methanol.

The pressure is not critical in the process of the invention. Thedehydrogenation of the methanol can be carried out at subatmosphericpressure, atmospheric pressure or superatmospheric pressure. A range offrom about 0.1 to 10 bar, preferably from 0.5 to 2 bar, is particularlyuseful. Preference is given to atmospheric pressure. The process of theinvention can be carried out batchwise or continuously, with preferencebeing given to the latter. The temperature is generally from 300° C. to950° C., preferably from 500 to 900° C., particularly preferably from600 to 850° C.

Preferably from 0.01 to 1 kg of methanol per hour and per gram ofcatalyst used is reacted. In the case of a continuous process, furthercatalyst has to be introduced continuously or discontinuously. Theamounts involved here are generally from 10 milligrams to 5 grams,preferably from 10 mg to 1 g, particularly preferably from 50 to 1000mg, very particularly preferably from 50 to 500 mg, per kg of methanolreacted.

The catalysts used can be, for example, those known from the literature,as are described, for example, in Chem. Eng. Technol. 1994, 17, 34.

Suitable metals are, for example, Li, Na, K, Cs, Mg, Al, In, Ga, Ag, Cu,Zn, Fe, Ni, Co, Mo, Ti, Pt, or their compounds. Also suitable are, forexample, S, Se, phosphates of transition metals such as V and Fe, andheteropolyacids such as molybdophosphoric acid.

Examples of specific catalysts are:

Sodium or sodium compounds (DE-A-37 19 055 and

DE-A-38 11 509)

Aluminum oxide, alkali metal aluminate and/or alkaline earth metalaluminate (EP-A-04 05 348)

Silver oxide (JP-A 60/089 441, Derwent Report 85-15 68 91/26)

A catalyst comprising copper, zinc and sulfur (DE-A 25 25 174)

A catalyst comprising copper, zinc and selenium (US-A 4,054,609)

A catalyst comprising zinc and/or indium (EP-A 0 130 068)

Silver (US-A 2,953,602)

Silver, copper and silicon (US-A 2,939,883).

Preference is given to using sodium or sodium compounds.

The form in which such a catalyst, for example a sodium-containingcatalyst, is used can vary widely:

Metallic, e.g. also as an alloy with at least one other alloyconstituent, as compound or salt, where at least one nonmetallic elementis chemically combined with Na (binary compounds and salts). If morethan one element is present in chemically combined form in the compound,a binary, ternary or quaternary compound or a salt is present.

If sodium is used in metallic form, it can be used as solid, liquid orpreferably as vapor.

Preferred alloys are those with other alkali metals and/or alkalineearth metals, for example Ba, Sr, Ca, Cs, Rb, K or, particularlypreferably, Li and/or magnesium.

Furthermore, alloys with B, Al, Si and Sn can also be used. This alsoapplies, in particular, to alloys which can comprise compounds such assodium boride, NaB₂, sodium silicide, NaSi or NaSn.

Examples of suitable binary sodium compounds and salts are sodiumcarbides such as Na₂C₂, NaC₈, sodium halides such as NaF, sodium oxidessuch as Na₂O, sodium azide, sodium phosphide, sodium sulfide, sodiumpolysulfides, preferably also sodium hydrides such as NaH.

Examples of suitable ternary sodium compounds and salts are sodiumborates such as borax, sodium phosphates or hydrogenphosphates, sodiumphosphites, sodium (meta)silicates and aluminosilicates, e.g. waterglass, Na₃AlF₆ (cryolite), sodium (hydrogen)sulfate, sodium sulfite,sodium nitrite, sodium nitrate, sodium amide, sodium acetylide NaCCH,sodium cyanide, sodium thiocyanate, the sodium salt of methyl thiol,sodium thiosulfate, but preferably NaOR, where R=H or an organic radical(=salts of organic acids, alkoxides, phenoxides, acetylacetonate,acetoacetic ester salt, salts of salicylic acid or salicylaldehyde),sodium carbonate and sodium hydrogencarbonate and mixtures thereof, forexample soda, thermonatrite, trona, pirssonite, natrocalcite. The use ofanhydrous, i.e. dried, salts is generally preferred.

Particular preference is given to NaOH, NaOOC—R (preferably formate,acetate, lactate, oxalate), NaOR=(R=is an organic radical having from 1to 4 carbon atoms) and sodium carbides.

Very particular preference is given to NaOH, sodium formate, sodiummethoxide, sodium acetate and sodium carbides such as Na₂C_(2.)

Suitable quaternary compounds are, for example, sodium-containingaluminosilicates which can be prepared synthetically or can also occurin a wide variety as natural minerals and rocks (e.g. sodium feldspar oralbite and calcium-sodium feldspar or oligoclase). They can additionallybe laden with Na by ion exchange.

Use can also advantageously be made of double salts of the alum type orthenardite, glauberite, astrakanite, glaserite, vanthoffite.

The sodium compounds and salts mentioned here can advantageously also bein the form of mixtures. In particular, it is also quite possible to usemixtures containing <50%, preferably <30%, of cations of other alkalimetals and/or alkaline earth metals, e.g. Ba, Sr, Ca, Cs, Rb, K orpreferably Li and/or magnesium. Industrially available, complex mixturessuch as soda lime, ground basic slag and cements, e.g. Portland cement,if desired after enrichment with sodium by storage in sodium-containingsolutions (NaCl, sea water) are particularly advantageous.

The abovementioned compounds used as catalysts give yields of over 70%at reaction temperatures of from 700 to 850° C. and low waterconcentrations of less than 5 mol% of H₂O per mole of formaldehyde.Advantages of the lower reaction temperature are the lower energy andapparatus requirements for heating/cooling before/after the reaction,the low decomposition rate of the formaldehyde which is thermallyunstable under the reaction conditions and the lower demands placed onthe materials of construction.

The abovementioned substances will hereinafter be described as theprimary catalyst.

The liberation of the catalytically active species from the primarycatalyst is preferably carried out by thermal decomposition of thelatter.

The primary catalyst can, for example, be introduced initially orafterwards, in each case continuously or discontinuously, as a solid,dissolved in a solvent, as a liquid or as a melt.

The further introduction of the primary catalyst as a solid, e.g. inpowder form, particulate or compacted, is generally carried out by meansof solids metering, e.g. using a reciprocating or rotary piston, a cellwheel lock, a screw or a vibrating chute.

If the primary catalyst is added in dissolved form, particularlysuitable solvents are those having a chemical composition consistingonly of the elements already present in the process (C,H,O). MeOH isparticularly preferred as solvent. The addition is carried out, forexample, via a nozzle which can be cooled in order to avoid evaporationof the solvent, crystallization or deposition of the solid primarycatalyst in the nozzle.

The addition of the primary catalyst as a melt can be carried out, forexample, via a nozzle. The melt can be vaporized or decomposed directlyin the gas stream.

In all possible ways of subsequent introduction of the primary catalyst,this is advantageously carried out in a manner such that the material isin intimate contact with flowing gas. This can be achieved, for example,by applying the catalyst material according to the above-describedprocesses to a suitable surface through or over which the gas flows.This can be the surface of a support material present as a fixed bed.Suitable materials are, for example, SiC, SiO₂ and Al₂O₃ in a suitablegeometric form, e.g. as granules, pellets or spheres. The material ispreferably arranged vertically in a fixed bed, preferably with meteringin from above. The substance introduced deposits on the support materialand the catalytically active species goes into the gas phase during theprocess.

Another possibility is arrangement of the primary catalyst in afluidized bed through which the carrier gas stream is passed. Thefluidized material comprises at least some of the supported orunsupported primary catalyst. The loss of active substance can bereplaced by further introduction of fresh primary catalyst, exhaustedmaterial can be taken off if desired. In a continuous process, this canbe realized, for example, by means of a circulating fluidized bed.

Subsequent introduction of the primary catalyst can also be carried outby alternating secondary catalyst generation in various vessels in whichthe primary catalyst can be arranged, for example as fixed bed orfluidized bed, in each case supported or unsupported.

The advantage of using a plurality of units for the discontinuoussubsequent introduction of catalyst is that it is also possible to usethose primary catalysts for which, e.g. owing to material propertiessuch as melting point, viscosity or decomposition temperature,continuous introduction would be impossible or very costly.

In a preferred variant of the process of the invention, the generationof the secondary catalyst is carried out physically separately from thereaction zone in which the actual dehydrogenation takes place and at atemperature above the dehydrogenation temperature.

The temperature difference between the location of catalyst generationand the reaction zone is preferably at least 20° C., particularlypreferably from 40 to 250° C.

On thermal treatment of the primary catalysts of the invention in theprimary catalyst decomposition zone and on passing over a reducing ornonreducing gas such as molecular nitrogen at temperatures which may bedifferent from the reaction temperature for the dehydrogenation and maybe higher or lower, one or more catalytically active species which areable to catalyze the dehydrogenation of methanol are released orgenerated from the primary catalyst and/or generated on it (secondarycatalyst). Such a fluid catalyst can be transported over considerabledistances without suffering from an appreciable loss of effectiveness inthe dehydrogenation. This separate setting of temperatures makes itpossible, in particular, to lower the reaction temperature by matchingto the respective conditions for catalyst liberation/varporization orgeneration of a catalytically active species (secondary catalyst) on theone hand and to the reaction on the other hand. This reduces thedecomposition of the formaldehyde, which is unstable under the reactionconditions, as a result of secondary reactions and increases the yield.

Preferred temperatures for generating the secondary catalyst from theprimary catalyst are from 300 to 1100° C.; particularly preferredtemperatures are from 400 to 1000° C.

In addition, the residence times in the dehydrogenation reactor andvessels for adding primary catalyst or for generating the secondarycatalyst can be set separately by dividing the carrier gas stream. Thisachieves targeted loading of the gas stream passed through the catalystaddition unit with the active species.

Preferred residence times for generating the secondary catalyst are from0.01 to 60 sec, particularly preferably from 0.05 to 3 sec.

If the generation of the primary catalyst is carried out physicallyseparately from the reaction zone, the temperatures in the reaction zoneare generally from 200 to 1000° C., preferably from 300 to 980° C.

For the dehydrogenation of the methanol, the residence time in thereaction zone is preferably from 0.005 to 30 sec, particularlypreferably from 0.01 to 15 sec, very particularly preferably from 0.05to 3 sec.

Suitable reactors are well known to those skilled in the art.Essentially, it is possible to use reactor types and assemblies as areknown from the literature for dehydrogenation reactions. Suchapparatuses are described, for example, in Winnacker/Kuchler, ChemischeTechnologie, 4^(th) Edition, Chapter “Technik der Pyrolyse” HanserVerlag, Munich 1981-86. Examples of suitable reactors are tube reactors;suitable reactor materials are, for example, ceramic materials such asa-alumina but also iron- and nickel-based alloys which are resistant tocarbonization, heat and scale, e.g. Inconel 600® or Hasteloy ®.

If the reactor is heated by means of a combustion reaction, a suitabletype of reactor is, for example, an externally fired tube reactor.

Preference is likewise given to heating the reactor by means ofmicrowaves.

In a further preferred variant of the process of the invention, acarrier gas stream at a temperature above the dehydrogenationtemperature is fed into the reactor.

The temperature difference between carrier gas stream anddehydrogenation temperature is preferably at least 20° C., particularlypreferably from 40 to 250° C.

The superheated gas stream can be fed directly into the reaction zone orall or some of it can be brought into contact with the primary catalystbeforehand.

For the superheated gas stream, the preferred temperatures are from 600to 1000° C., particularly preferably from 700 to 900° C. Preferredtemperatures for the dehydrogenation of methanol are from 500 to 900°C.; particular preference is given to temperatures of from 600 to 800°C.

The carrier gas stream or streams can consist of a reducing ornonreducing gas, e.g. H₂/CO mixtures or nitrogen, preferably theby-products of the dehydrogenation.

Such a process is subject matter of the German Patent Application 197 22774.0 which is hereby expressly incorporated by reference into thepresent description.

The formaldehyde can be separated from the reaction mixture by methodsknown per se with which those skilled in the art are familiar, forexample by condensation, polymerization or physical or chemicalabsorption or adsorption.

An industrially proven method is the formation of hemiacetals fromformaldehyde and an alcohol. The hemiacetals are subsequentlydissociated thermally, giving very pure formaldehyde vapor. The alcoholused is usually cyclohexanol since its boiling point is sufficiently farabove the decomposition temperature of the hemiacetal. The hemiacetalsare usually dissociated in falling film or thin film evaporators attemperatures of from 100 to 160° C. (see, for example, U.S. Pat No.2,848,500 of Aug. 19, 1958 “Preparation of Purified Formaldehyde” andU.S. Pat. No. 2,943,701 of Jul. 5, 1960 “Process for purification ofgaseous formaldehyde”, or JP-A 62/289 540). The formaldehyde vaporsliberated in this dissociation still contain small amounts of impuritieswhich are usually removed by countercurrent scraping with alcohol suchas cyclohexanol hemiformal, by condensation or by controlledprepolymerization.

Particularly preferred methods of purifying the formaldehyde preparedaccording to the invention are described in the German PatentApplications 19 747 647.3 and 19 748 380.1.

A further method of separating formaldehyde from the reaction mixture isthe formation of trioxane in a catalytic gas-phase process (see, forexample, Appl. Catalysis A 1997, 150, 143→151 and EP-A 0 691 338).Trioxane can then, for example, be condensed out.

Further possible ways of utilizing the by-products of the reaction, inparticular hydrogen, are, for example, the synthesis of methanol or theisolation of pure hydrogen which can, for example, be separated off bymeans of membranes.

The hydrogen obtained in this way is suitable, for example, for thesynthesis of ammonia, in refinery processes for producing gasoline andpetrochemical cracking products, for the synthesis of methanol, forhardening fats and for other hydrogenations, as reducing agent forproducing W, Mo, Co and other metals, as reducing protective gas inmetallurgical processes, for autogenous welding and cutting, as fuel gasin admixture with other gases (town gas, water gas), or in liquefiedform as fuel in aerospace applications.

The formaldehyde prepared by the process of the invention is suitablefor all known fields of application, for example corrosion protection,production of mirrors, electrochemical coatings, for disinfection and asa preservative, also as intermediate for preparing plastics, for examplepolyoxymethylenes, polyacetals, phenolic resins, melamines,amino-plastics, polyurethanes and casein plastics, 1,4-butanols,alcoholic formaldehyde solutions, methylal, trimethylolpropane,neopentyl glycol, pentaerythritol and trioxane, for producing dyes suchas fuchsin, acridine, for producing fertilizers and for the treatment ofseed.

The invention also relates to plastics such as polyoxymethylene andpolyacetals, trioxane, dyes, fertilizers and seed produced in this way.

The invention further provides a process for preparing trioxane, whichcomprises

1. converting methanol into formaldehyde by dehydrogenation in a reactorin the presence of a catalyst at a temperature in the range from 300 to1000° C., where a circulating gas stream comprising by-products of thedehydrogenation is passed through the reactor, and

2. if desired, purifying the formaldehyde prepared in this way andtrimerizing it to give trioxane.

Details of the preparation of trioxane are well known to those skilledin the art. They are described, for example, in Kirk-Othmer,Encyclopedia of Chemical Technology, 2nd Edition, Volume 10, pp. 83, 89,New York lnterscience 1963-1972.

The invention likewise provides a process for preparingpolyoxymethylene, which comprises

1. converting methanol into formaldehyde by dehydrogenation in a reactorin the presence of a catalyst at a temperature in the range from 300 to1000° C., where a circulating gas stream comprising by-products of thedehydrogenation is passed through the reactor, and

2. if desired, purifying the formaldehyde obtained in this way,

3. polymerizing the formaldehyde,

4. capping the end groups of the polymer prepared in this way and

5. if desired, homogenizing the polymer in the melt and/or providing itwith suitable additives.

The preparation of polyoxymethylene from formaldehyde is well known tothose skilled in the art. Details may be found, for example, inUllmann's Encyclopedia of Industrial chemistry, Volume 21, 5th Edition,Weinheim 1992 and the literature cited therein.

The invention further provides a process for preparing polyoxymethylenecopolymers, which comprises

1. converting methanol into formaldehyde by dehydrogenation in a reactorin the presence of a catalyst at a temperature in the range from 300 to1000° C., where a circulating gas stream comprising by-products of thedehydrogenation is passed through the reactor, and

2. trimerizing the formaldehyde obtained in this way to give trioxane,

3. if desired, purifying the trioxane,

4. copolymerizing the trioxane with cyclic ethers or cyclic acetals,

5. if desired, removing unstable end groups and

6. if desired, homogenizing the polymer prepared in this way in the meltand/or admixing it with suitable additives.

The invention further provides a process for preparing polyoxymethylenecopolymers, which comprises

1. converting methanol into formaldehyde by dehydrogenation in a reactorin the presence of a catalyst at a temperature in the range from 300 to1000° C., where a circulating gas stream comprising by-products of thedehydrogenation is passed through the reactor, and

2. if desired, purifying the formaldehyde obtained in this way,

3. copolymerizing the formaldehyde with cyclic ethers or cyclic acetals,

4. if desired, removing unstable end groups and

5. if desired, homogenizing the polymer prepared in this way in the meltand/or admixing it with suitable additives.

The preparation of polyoxymethylene copolymers is well known to thoseskilled in the art. Details may be found, for example, in Ullmann'sEncyclopedia of Industrial Chemistry, Volume 21, 5th Edition, Weinheim1992 and the literature cited therein, and also in the Russian documentsSU 436067, 740715 and SU 72-1755156, 720303.

The contents of the priority-establishing German Patent Applications 19722 774.0, 197 27 519.2, 197 27 520.6 and 197 43 145.3 and also theabstract of the present application are expressly incorporated byreference into the present description.

The invention is illustrated by the examples without being limitedthereby.

EXAMPLES

The conversion and yield are calculated as follows:${{conversion}\quad \left( {{in}\quad \%} \right)} = {\frac{{methanol}\quad {reacted}\quad ({mole})}{{methanol}\quad {fed}\quad {in}\quad ({mole})} \cdot 100}$${{yield}\quad \left( {{in}\quad \%} \right)} = {\frac{{formaldehyde}\quad {formed}\quad ({mole})}{{methanol}\quad {fed}\quad {in}\quad ({mole})} \cdot 100}$

Experiments on the dehydrogenation of methanol which were carried out ina microwave-heated and in an electrically heated laboratory reactor aredescribed below. FIG. 2 shows a schematic flow diagram of theconfiguration of the microwave-heated laboratory reactor, FIG. 3 showsthat of the electrically heated laboratory reactor.

A. Reactor Heated by Microwaves

The actual reaction space is the interstitial space in a bed 11. The bed11 comprises SiC spheres having a diameter of a few millimetres and islocated in a quartz reactor 12. For heating, the reactor 12 is installedin a microwave heating chamber 13 where heat is liberated in the SiCspheres as a result of irradiation. This method of reactor heating makesit possible to achieve a significantly more homogeneous temperaturedistribution than, for example, in externally heated tubes. Thetemperature is measured in the bed and is set by regulating (TIC) thepower of the radiation.

A carrier gas 15 (preferably nitrogen) flows through the bed 11 from thebottom upward. Methanol 16 is introduced through a vertical tube 17extending to about half the height of the bed 11 and there goes througha frit 18 into the bed 11 where it mixes with the carrier gas 15.Primary catalysts 19 in tho form of grains are introduced into the lowerquarter of the bed 11. Apart from the methanol 16, either the carriergas 15 is fed into the reactor 12 or the reaction products afterseparating off the formaldehyde 21 in a cold trap 22 are recirculated tothe reactor 12 as circulating gas 20. Excess reaction gas and/or carriergas 15 can be discharged under pressure control via a valve 23.

The total volume flow is from 20 l/h to 500 l/h, the residence timewithin the bed varies from 0.02 to 1 s and the proportion of methanol isfrom 5 to 50 mol%. As solid, various alkali metal compounds are used.The reaction product is analyzed by means of a gas chromatograph.

TABLE 1 Conversion and formaldehyde yield in the pyrolysis of methanol(residence time at the reaction temperature about 0.2 s, inletconcentration of methanol about 10 mol %, amount of catalystcorresponding to about 0.5 g of alkali metal) Example/ TemperatureConversion Yield in comparative Solid in of Yield circulating example(catalyst) the bed methanol in N₂ gas Ex. 1 none approx. 910° C. 60% 12%CE 1 none approx. 920° C. 70% 11% Ex. 2 Na₂CO₃ approx. 880° C. 93% 67%CE 2 Na₂CO₃ approx. 880° C. 92% 63% Ex. 3 NaOH — 80% 54% Ex. 4 NaCHO₂approx. 790° C. 91% 65% CE 3 NaCHO₂ approx. 820° C. 83% 52% Ex. 5NaC₂H₃O₂ approx. 780° C. 92% 63% CE 4 NaC₂H₃O₂ approx. 760° C. 84% 55%Ex. 6 LiOCH₃ approx. 780° C. 91% 39% CE 5 LiOCH₃ approx. 795° C. 95% 32%Ex. 7 Cs₂CO₃ approx. 865° C. 90% 26% CE 6 Cs₂CO₃ approx. 900° C. 90% 22%Ex. 8 KOCH₃ approx. 845° C. 88% 27% CE 7 KOCH₃ approx. 840° C. 88% 20%Ex. 9 Na₂C₂ approx. 765° C. 77% 58% Ex. 10 Na₂C₂ approx. 775° C. 89% 58%CE 8 Na₂C₂ approx. 805° C. 80% 53% Ex. 11 Na₂(COO)₂ approx. 785° C. 87%51% Ex. 12 NaOCH₃ approx. 760° C. 91% 70% CE 9 NaOCH₃ approx. 760° C.89% 62%

TABLE 2 Conversion and formaldehyde yield in the pyrolysis of methanol(residence time at the reaction temperature about 0.1 s, inletconcentration of methanol about 10 mol %, amount of catalystcorresponding to about 0.5 g of alkali metal) Example/ TemperatureConversion Yield in comparative Solid in of Yield circulating example(catalyst) the bed methanol in N₂ gas CE10 NaC₂H₃O₂ approx. 785° C. 92%76% Ex. 13 NaC₂H_(3O) ₂ approx. 805° C. 92% 69% Ex. 14 Na₂C₂ approx.780° C. 89% 67% Ex. 15 Na₂C₂ approx. 765° C. 83% 68% Ex. 16 Na₂(COO)₂approx. 775° C. 94% 68% CE 11 Na₂(COO)₂ approx. 765° C. 81% 62%

B. Electrically Heated Reactor

The dehydrogenation of the methanol 16 is carried out in a tube reactor24 heated by means of electrical laboratory furnaces (see FIG. 3). Thecatalyst 25 (0.1-5 g) is placed in a tube in the first furnace 24 a(700-1000° C.) and circulating gas 26 is passed over it. Downstream ofthis, the preheated methanol 16 (20-500 g/h, 400-800° C.) is fed in.This is followed by the actual reaction zone 24 b (tube; length of 20-50cm, internal diameter 7-20 mm) in a second furnace (700-1000° C.).Immediately after the reactor, the reaction products are cooled to about150° C. In a column 27, the reaction products are scrubbed with alcohol28 (e.g. cyclohexanol at 20-80° C.) in order to remove 21, 28 theformaldehyde 21. After the scrub, the excess 29 of the gaseous productsis discharged under pressure control (PIC) by means of a valve 30 andthe remainder is recirculated as circulating gas 26 (100-2000 l/h) andis preheated in the first furnace 24 a.

The use of

80 g/h of methanol

400 l/h of circulating gas

1 g of sodium methoxide as catalyst

gives 54 g/h of formaldehyde.

What is claimed is:
 1. A process for preparing formaldehyde frommethanol by dehydrogenation in a reactor in the presence of a catalystat a temperature in the range from 300 to 1000° C., wherein acirculating gas stream comprising by-products of the dehydrogenation ispassed through the reactor.
 2. The process as claimed in claim 1,wherein the circulating gas stream consists essentially of H₂ and CO. 3.The process as claimed in claim 2, wherein the molar ratio H₂/CO in thecirculating gas is
 3. 4. The process as claimed in claim 1, wherein thereactor is an externally fired tube reactor.
 5. The process as claimedin claim 1, wherein the reactor is heated by means of microwaves.
 6. Theprocess as claimed in claim 1, wherein a further part of the by-productsof the dehydrogenation is used as fuel for heating the reactor.
 7. Theprocess as claimed in claim 1, wherein the catalyst used is sodium or asodium compound which contains, apart from Na, only elements selectedfrom the group consisting of C, H and O.
 8. An apparatus for carryingout a process as claimed claim 1, comprising a heat exchanger forpreheating the starting materials, a reactor for carrying out thedehydrogenation, a heat exchanger for cooling the product mixture, aseparation vessel for separating off the formaldehyde and means forrecirculating at least part of the by-products of the dehydrogenation(circulating gas) to the reactor.
 9. An apparatus as claimed in claim 8which comprises means for discharging a further part of the by-productsof the dehydrogenation and feeding this part to an apparatus for heatingthe reaction in which latter apparatus it serves as fuel.
 10. Theprocess as claimed in claim 1, wherein part of hydrogen obtained asby-product is separated off.
 11. A process for preparing trioxane, whichcomprises converting methanol into formaldehyde by dehydrogenation in areactor in the presence of a catalyst at a temperature in the range from300 to 1000° C., where a circulating gas stream comprising by-productsof the dehydrogenation is passed through the reactor, and trimerizingthe formaldehyde to give trioxane.
 12. A process for preparingpolyoxymethylene, which comprises converting methanol into formaldehydeby dehydrogenation in a reactor in the presence of a catalyst at atemperature in the range from 300 to 1000° C., where a circulating gasstream comprising by-products of the dehydrogenation is passed throughthe reactor, and if desired, purifying the formaldehyde, polymerizingthe formaldehyde, capping the end groups of the polymer and if desired,homogenizing the polymer in the melt and/or providing it with additives.13. A process for preparing polyoxymethylene copolymers, which comprisesconverting methanol into formaldehyde by dehydrogenation in a reactor inthe presence of a catalyst at a temperature in the range from 300 to1000° C., where a circulating gas stream comprising by-products of thedehydrogenation is passed through the reactor, and trimerizing theformaldehyde to give trioxane, if desired, purifying the trioxane,copolymerizing the trioxane with cyclic ethers or cyclic acetals, ifdesired, removing unstable end groups and if desired, homogenizing thepolymer in the melt and/or admixing it with additives.
 14. A process forpreparing polyoxymethylene copolymers, which comprises, convertingmethanol into formaldehyde by dehydrogenation in a reactor in thepresence of a catalyst at a temperature in the range from 300 to 1000°C., where a circulating gas stream comprising by-products of thedehydrogenation is passed through the reactor, and if desired, purifyingthe formaldehyde, copolymerizing the formaldehyde with cyclic ethers orcyclic acetals, if desired, removing unstable end groups and if desired,homogenizing the polymer in the melt and/or admixing it with additives.